Hachinosu, also known as Pirate Island (海賊島,Kaizoku-jima?), is a New World island known to be a "Pirate Paradise" (海賊達の楽園,Kaizoku-tachi no Rakuen?) and is currently occupied by the Blackbeard Pirates.
Hachinosu is a relatively small-to-mid-sized island (still shown to dwarf Thriller Bark) whose foundation consists of dark-gray rock. Centrally on it are constructions made of yellowish rock, most prominently a large, skull-fashioned formation in its center, towering over all else. A city of many smaller buildings (all square, crudely brick-built ones) surrounds said skull, as does a series of proportionally huge palm trees.
The skull's inside is carved into what appears to be a fortress (where Blackbeard has been shown to reside), with the "nose" hole being a giant glass window (with additional, smaller windows dotting the skull here and there), and both "eye" holes going right through the whole structure. On the outside, the skull is carved to look like it is wearing a bandana, including a rocky protrusion on the left side shaped like the ends of a knot.
"Hachinosu" (蜂の巣,"Hachinosu"?) is Japanese for "beehive" or "honeycomb". It is also the colloquial term used to describe riddling someone with bullet holes, hence the term "Fullalead" in the Viz translation.
Hachinosu is likely inspired by real-world places:
Port Royal, Jamaica, another such haven where Caribbean pirates from around the world congregated during the 1600s. By the 1660s, the city had become a pirate utopia and gained a reputation as the "Sodom of the New World". Additionally, it has been historically subject to severe earthquakes plus accompanying tsunamis, which may be played on in-series with Marshall D. Teach's use of the Gura Gura no Mi.
Hachinosu is similar to Onigashima, as both islands are most distinguished by a massive, skull-shaped structure in their center, and both are occupied by one of the Four Emperors (Blackbeard and Kaido respectively).